1. Field of the Invention
The invention relates to a method for producing a molding sand, particularly one that is recirculated, for foundry purposes, according to which a mixture of a granular substance and additives, such as a binder and water, are added to a material that is not capable of swelling in water.
2. The Prior Art
It is known that in foundry technology, synthetic molding sands are used, which essentially consist of the refractory, granular basic substance, i.e. granular mass, and additives. The additives have various tasks. In particular, they are supposed to contribute to an improvement of the surface quality of the cast parts produced using the molding sand described. Binders are added to the molding sands as additives. These binders are usually natural inorganic binders, such as clays, particularly clays that contain montmorillonite, so-called bentonites.
Bentonites as molding sand binders are connected with the advantage that the molding sand can be recirculated, and that in this regard, treatment of the thermally stressed molding sand is possible. This presumes, of course, that its crystalline structure has not been destroyed by heat, which does or can apply for approximately 0.5 wt.-% of the bentonite per circulation cycle.
Treatment of the molding sand generally takes place by means of mixing water into the molding sand again, whereby approximately 1 wt.-% of additives (including bentonite) must be added. This can be attributed to the thermal burn-off at the contact surface of the molding sand, i.e. mold, with the cast part. During the molding substance or molding sand treatment, the molecules of water deposit in and on the binder (bentonite), causing the binder to form the ability to be able to bind the otherwise granular basic substance, i.e. granular mass. A measure of this binding ability and of the resulting strength properties of a compacted molding sand is, among other things, the so-called wet tensile strength, which is measured in N/cm3. The wet tensile strength can be determined as indicated in EP 0 644 006 A1 and the VDG-Merkblatt [bulletin of the Verein Deutscher GieBereifachleute=Association of German Foundry Specialists] which is cited there.
In today's foundries, thermal stress on the molding sand or molding substance is great, furthermore the treatment times are short, and a significant influx of core sand, i.e. granular mass takes place. In order to reduce emissions, an effort is made to use little so-called glance carbon forming agents, e.g. hard coal dust. Accordingly, a molding sand having a low wash substance content as well as a low content of coking residues is obtained.
The low wash substance content expresses the fact that the proportion of particles having a low diameter of mostly less than 20 μm is small. As a result, the molding sand or molding substance becomes sensitive to water, in other words it reacts particularly strongly to changes in the amount of added water, so that control of the compaction of the mold sand becomes problematic.
Within the scope of EP 0 644 006 A1, a binder for foundry mold sand is described. This binder can be produced on the basis of alkali-activated earth alkali smectites. The earth alkali smectite, in each instance, is activated in the alkali form by means of an inorganic ion exchanger. Usually, the earth alkali smectite is calcium bentonite or calcium-magnesium bentonite. The problems of emission reduction as indicated above have not been significantly influenced by this state of the art. This also holds true for the method known from WO 98/17 417 A1, for the production of a molding sand for foundry purposes.